Why do submarines move more like torpedoes than fish?

While a fish’s low-drag scales and body shape make it better suited at sliding through the water than submarines, fish do not have the advantage of a rotating machine behind them…

By Deborah Halber

While a fish’s low-drag scales and body shape make it better suited at sliding through the water than submarines, fish do not have the advantage of a rotating machine behind them.

“There’s nothing simpler to Industrial Age people than building a propulsion device — an electric motor, an internal combustion engine or a turbine — connected to a propeller,” said Franz S. Hover, assistant professor of mechanical engineering. “The propeller on a submarine a lot like a propeller on a plane — the blade is angled, accelerating the water or air in one direction. It’s this acceleration that creates the thrust that moves the submarine forward.” (Unlike torpedoes, whose supercharged propellers are only meant to last long enough for them to connect with their targets, submarines have much more efficient, quieter propellers and motors.)

In contrast, “imagine building an eel with rotary motors,” he said. To engineers, fish and other undulating creatures have “an unsteady configuration. It would take complicated linkages and sophisticated robotic design to design a submarine that moved like a fish,” Hover said. “We wouldn’t know how such a contraption would fatigue or fail, and until we do, it’s unlikely people would want to let it carry them miles beneath the sea.”